1. Field of the Invention
This invention relates to a method of and an apparatus for controlling a vacuum modulating valve for the exhaust gas recirculation control, and more particularly to improvements in a method of and an apparatus for controlling a vacuum modulating valve for the exhaust gas recirculation control, incorporating therein a temperature detecting element, suitable for use in an exhaust gas recirculation system (hereinafter referred to as an "EGR system") in an electronically controlled diesel engine for a motor vehicle.
2. Description of the Prior Art
In Japanese Patent Laid-Open No. 67954/1983, for example, there has been proposed such a technique that there is provided a vacuum modulating valve for modulating the control vacuum of an exhaust gas recirculation value control valve (hereinafter referred to as an "EGR valve") in order to control the exhaust gas recirculation (hereinafter referrd to as "EGR") in a diesel engine, particularly in a diesel engine for a motor vehicle. An electric signal commensurate to the operating conditions of the engine is applied to a solenoid of the vacuum modulating valve, whereby the control vacuum supplied to a diaphragm chamber of the EGR valve is made to be a value such that a predetermined exhaust gas recirculation rate (hereinafter referred to as an "EGR rate") commensurate to the operating conditions of the engine can be obtained.
According to this conventional technique, a duty ratio is calculated from the predetermined EGR rate, and a signal of the duty ratio is applied to the solenoid of the vacuum modulating valve. Because of this, a normal EGR rate characteristic suited to the operating conditions of the engine can be obtained. In this case, the provision of a feedback circuit for causing a coil current to be constant for the temperature compensation of the solenoid makes it possible to avoid a shift of the EGR rate if heat is generated by the actuation of the solenoid. However, integrated circuits (hereinafter referred to as "IC") are in needed in number due to the current feedback circuit, thus presenting the disadvantages of increased number of parts and raised cost.
On the other hand, to solve the above-described disadvantages, the applicant has proposed in Japanese Patent Application No. 66672/1984, that a temperature detecting element such as a thermistor is incorporated in the vacuum modulating valve, and a required drive signal of the vacuum modulating valve is calculated in a temperature compensating manner by a temperature signal from the temperature detecting element, whereby the temperature compensation of the current flowing through a coil in the vacuum modulating valve is conducted with no use of a current feedback circuit.
However, according to the aforesaid senior application, the drive signal of the vacuum modulating valve is corrected only by a temperature outputted from the thermistor. Therefore, if this correction is not satisfactory, the coil current is not accurately corrected, the vacuum modulating characteristics of the vacuum modulating valve is shifted from the required characteristic and the EGR value is shifted from the requirement, thus possibly causing lowered output power of the engine, generation of white smoke or smoke, deteriorated exhaust emission and the like.